Despite the widespread acceptance of harmonic imaging in ultrasound, the mechanisms by which image quality is improved over conventional ultrasonography are not well understood. Three such mechanisms have been proposed in the literature, and these mechanisms have been demonstrated in water tank experiments. However, the significance of these mechanisms remain ambiguous for in vivo imaging. We hypothesize that elucidation of these mechanisms can lead to optimization of harmonic signal generation and yield further image quality improvement. We have developed powerful simulation tools capable of propagating nonlinear waves through various media including diffuse scatterers and layered and aberrating media in 3-D. These simulations are not limited to forward propagating waves, and can model mul- tipath reflections from diffuse scattering sites and reverberations from layered and aberrating media. We have also developed in vivo experimental techniques for characterizing each mechanism individually. We have developed these techniques to be used on a novel 1728 channel beamforming system capable of real-time 3-D imaging. We propose a series of experiments designed to separate and determine the contribution of each mechanism in vivo. These experiments are performed in the bladders and livers of volunteers and patients using the 1728 channel beamforming system. In addition, we propose novel simulation studies, informed by our in vivo experimentation, that are capable of 3-D nonlinear wave propagation. Based on our simulations and clinical studies, we propose optimization methods of harmonic signal generation and a second round of clinical studies to observe the optimized harmonic imaging methods in vivo. PUBLIC HEALTH RELEVANCE: In this proposal, we address the physical mechanisms of harmonic imaging in ultrasound. The key problem is that these mechanisms are poorly understood when imaging the human body. We propose to expand our knowledge of these mechanisms and improve harmonic imaging methods based on this knowledge.